I don't suppose there's any easy way to use an automatic jpg thumbnail....... but only if the browser appears not to have an appropriate plug-in for displaying tiffs inline.
The DjVu and PDF handlers provides thumbnails as jpgs.
I would expect that the TIFF handler will also do this, otherwise it will not integrate with the Proofread Page extension.
On a related note is there any chance of getting the ability to specify thumbnail formats when adding an image to a page and/or determining what format to use huristically? We already support png which is a lossless format with wide support and better compression than TIFF but if you upload a photo as a png at the moment the thumbnails will also be pngs which is generally bad.